Authors of physicstextbooks frequently use the deflection of a thin, vertically falling water jet by a charged balloon,1–3 comb,4–6 or rod7–9 as a visually appealing and conceptually relevant example of electrostatic attraction. Nevertheless, no attempts are made to explore whether these charged bodies could cause visible deformation of a horizontal water surface. That being so, we were quite surprised when we discovered that a 19th-century French book10 contained a drawing showing an appreciable deformation of an oil surface caused by a charged rod. When we initially tried to recreate this electrostatics demonstration, we didn't succeed in reproducing the effect with a charged rod. Despite the initial unsuccessful try, we were not discouraged and we modified the demonstration a little bit, finding that it was possible to cause visible deformations of different liquid surfaces by using a Van de Graaff generator, as we will explain later.

Physics students encountering electric circuits for the first time often ask why adding more resistors to a circuit sometimes increases and sometimes decreases the resulting total resistance. It appears that these students have an inadequate understanding of current flow and resistance. Students who do not adopt a model of current, voltage, and resistance necessarily resort to memorizing formulas for calculating, e.g., the resistance of a resistor network. For these students, certain properties of electric circuits may remain mysterious or puzzling.

Since 1987, the Statistical Research Center at the American Institute of Physics has regularly conducted a nationwide survey of high schoolphysics teachers to take a closer look at physics in U.S. high schools.1 We contact all of the teachers who teach at least one physics course at a nationally representative sample of all U.S. high schools—both public and private schools. Our most recent survey was conducted during the 2012–13 school year. While our questionnaire covers a number of areas of interest, in this article we examine the number of students enrolled in high schoolphysics courses and the types of courses offered. We also take a closer look at the prior physics experience of students enrolled in Advanced Placement (AP) Physics classes.

The classic experiment to measure the drag coefficient involves dropping coffee filters.1 Wouldn't it be more fun to try something different? In fact, an experiment on the drag force is conducted nearly 4000 times a day during the baseball season and you have free access to this PITCHf/x data!2

Acommon undergraduate laboratory experience is the determination of the elastic constant of a spring, whether studying the elongation under a static load or studying the damped harmonic motion of the spring with a suspended mass. An alternative approach to this laboratory experience has been suggested by Menezes et al.,1 aimed at studying the dependence of the elastic constant with the length of the spring. The proposal by Menezes et al.1 consists of determining the springs elastic constant K (defined as the ratio of the elongation δ and the magnitude of applied force F) using the usual method of suspending a mass m and studying the dependence of K on the springs unstretched length L. The authors vary the length of a spring by cutting pieces off the end and obtain an experimental relation , with α a constant depending on the spring materials and geometrical factors. In our teaching practice we have been using the experience in Ref. 1 as an opportunity for advanced students to earn extra credit after the ordinary laboratory experience. The results we have obtained so far confirm the work of Menezes et al.1 and inevitably motivate a discussion on the dependence of the elastic constant K with some other spring parameters. The present work originates from the quest of a couple of students to determine the role of the spring coil diameter D on the value of K and α.

This paper is an addition to the series of papers on the exponential function begun by Albert Bartlett.1 In particular, we ask how the graph of the exponential function would appear if y were plotted versus ln t rather than the normal practice of plotting ln y versus t. In answering this question, we find a new way to interpret the mean life (or time constant) τ using such a linear-log graph.

This is a companion to our previous paper1 in which we give a published example, based primarily on Perry's work,2,3 of a graph of ln y versus t when y is an exponential function of t. This work led us to the idea that Lord Kelvin's (William Thomson's) estimate of the Earth's age was wrong not because he did not account for radioactivity, as is commonly believed,4 but because he used the wrong model for Earth's heat loss. We feel this idea is worth spreading. To this end (following England et al.2,3), we examine two questions, the first about the radioactivity part and the second about Perry's alternate model for Earth's heat loss.

Energy Theater is a dynamic, full-body activity that engages all students in representing the flow of energy in various phenomena, such as a light bulb burning steadily or a refrigerator cooling food.1,2 In Energy Theater, each participant acts as a unit of energy that has one form at a time. Regions on the floor correspond to objects in a physical scenario, and participants move from one region to another to demonstrate the flow of energy among objects. (See Figs. 1, 3, and 4.) The goal of Energy Theater is for students to track energy transfers and transformations in real-world energy scenarios while employing the principle of energy conservation and disambiguating matter and energy. Unlike most representations of energy, which are static before-and-after accounting schemes for energy changes, Energy Theater is a dynamic representation that provides a natural stepping stone toward the more advanced ideas of energy density, energy current, and a continuity equation relating them. The fact that conservation of energy is embedded in the representation encourages students to “find the energy” in situations where it may be imperceptible. The rules of Energy Theater are listed in Fig. 2.

How do you motivate students to do their homework? Some instructors make students' homework scores a significant percentage of the final course grade. In that case, how much course credit is required? Some instructors do not grade homework at all, instead relying on students' intrinsic motivation to learn the course material. Will this actually work? Some instructors might motivate students by having quiz and/or exam problems closely match the assigned homework problems. In this article, we report on the effectiveness of grade incentives, homework-based quiz problems, and intrinsic motivation for 16 semesters of introductory mechanics and introductory electricity and magnetism (E&M) courses at the United States Air Force Academy (USAFA) between fall 2008 and spring 2012.

Universities and even high schools are moving more and more to online instruction as a cost-effective way to reach more students with fewer resources. This naturally raises the question: Can online learning be effective? (The question is not “Is online learning effective?” because just like face-to-face instruction, online instruction includes a diverse array of techniques.) In this paper I compare online and flipped face-to-face versions of an introductory astronomy course. Both versions were designed around student-centered learning principles, but the specific implementation of these principles varied according to the strengths of each type of instruction. Normalized Hake gains on the Star Properties Concept Inventory (SPCI) were quite similar for both classes: 56% and 58% for the online and flipped face-to-face versions, respectively. The gains obtained by students with low pre-test scores were as good as the ones achieved by students with high pre-test scores.